Petrogenesis of Early Paleozoic I-type granitoids in the Longshoushan and implications for the tectonic affinity and evolution of the southwestern Alxa Block

Abstract

Abstract. In the Paleozoic, the Alxa Block was situated between the Central Asian Orogenic Belt and the North Qilian Orogenic Belt, and experienced intense magmatic activity. Thus, the Alxa Block is an important area for understanding the tectonic framework and evolution of these two orogenic belts. However, there has long been debate regarding the tectonic affinity and tectonic evolution of the Longshoushan, located in southwestern margin of the Alxa Block, during the Paleozoic. In this study, we present zircon U-Pb ages, whole-rock major and trace elements, and Hf isotopic data for the granitoids from the east of the Longshoushan to investigate these issues. Bulk-rock analyses show that these granitoids are weakly peraluminous, with high SiO₂ and K₂O, but low MgO, TFe₂O₃ and P₂O₅. They are also characterized by enrichment in LREE and LILE, depletion in HREE and HFSE, and a large range of variation in εHf(t) values (monzogranite: -0.37 to -16.28; K-feldspar granite: 3.53 to -7.74). These geochemical features indicate that these granitoids are highly fractionated I-type granite, which were formed by crust and mantle-derived magma mixing. LA-ICP-MS zircon U-Pb dating constrains that the monzogranite and K-feldspar granite were formed at 440.8 ± 2.1 Ma and 439.4 ± 2.0 Ma, respectively. Combining these results with previous chronological data, the geochronology framework of Paleozoic magmatic events in the Longshoushan is consistent with the North Qilian Orogenic Belt to the south, but significantly differs from other parts of the Alxa Block and the Central Asian Orogenic Belt to the north. This result indicates that the Longshoushan was primarily influenced by the North Qilian Orogenic Belt during the Early Paleozoic. Integrated with previous studies, a three-stage tectonic model is proposed of Early Paleozoic accretion and arc magmatism leading to collision in the Longshoushan. (1) 460–445 Ma: Arc magmatism on an active continental margin with the northward subduction of the North Qilian back-arc basins (NQ bab). (2) 445–435 Ma: Magmatic rocks, dominated by I-type granites, were formed in a continent-continent collision setting. Significant crustal thickening is interpreted to result from compressional stress and/or magmatic additions. (3) 435–410 Ma: The development of abundant A-type granites and mafic dikes in response to intraplate extension, supported by a change in trace element chemistry indicating crustal thinning at this stage. This sequence of events and their timings is similar to other parts of the Central China Orogenic Belt, and requires either a coincidence of several oceanic plates closing at the same time, or an along-strike repetition of the same system.